Field of the Invention
The present invention generally relates to growing perovskite film layers.
Description of the Related Art
Among different alternative energy technologies, thin-film photovoltaics (PV) hold great promise to provide cleaner and sustainable energy at a cost competitive with fossil fuels provided the associated material and fabrication costs for manufacturing such photovoltaic modules get substantially lower at reasonable efficiency levels. Current state-of-the-art commercial photovoltaic devices are produced in non-continuous batch-to-batch processes at high temperatures using high-vacuum deposition methods. The associated capital cost for such processes is enormous. The foundation of the next generation PV technology is believed to be based on solution-processing of device components. Solution processed methylammonium lead tri-halide (CH3NH3PbX3, X═Cl, Br, I) perovskite as a photoactive starting material is of particular interest due to its earth abundant nature, low temperature processability, favorable electronic properties, high photovoltaic power conversion efficiencies (PCEs), and low-cost roll-to-roll (R2R) coating compatibility on large area flexible substrates. However, the utility of this material system towards successful technology deployment will only be possible through the combined efforts of improved materials engineering, fine control of the photoactive layer morphology and integration into sophisticated device architectures.
Contemporary perovskite solar cells are based on two main device architectures; namely a mesostructured configuration and a thin-film planar heterojunction structure. In both cases, high PCEs have been achieved for small area devices. The construction of complex mesostructured device architectures require high-temperature sintering (>450° C.) for the formation of electron-transporting metal-oxide layers, such as mesoporous or compact TiO2, which limits their applicability on flexible roll-to-roll compatible plastic substrates. Thin-film planar heterojunction (PHJ) structures, with no mesoporous TiO2, are advantageous for high-throughput manufacturing in terms of their simple device configuration and low temperature processing. Several planar-heterojunction structures (p-i-n and n-i-p), which avoid the mesoporous scaffold and have different combinations of charge transporting interlayers, have been investigated by numerous research groups and the PCEs from these systems are on par with those utilizing a mesostructured configuration. Planar heterojunction p-i-n structures consisting of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a hole-transport layer (p-type) and phenyl-C61-butyric acid methyl ester (PCBM) as an electron-transport layer (n-type) are promising due to their low-temperature solution processability, excellent bendability and tunable conductivity.
A typical perovskite based p-i-n planar heterojunction solar cell device fabrication starts using glass as a substrate, indium-doped tin oxide (ITO) as a transparent conductive oxide front contact and PEDOT:PSS as a hole-transport layer. Then the perovskite active layer is deposited on top of PEDOT:PSS, followed by a thin layer of PCBM as an electron acceptor and finally an aluminum (Al) metal layer as a cathode. At present, one of the main issues encountered in this device fabrication process is the fine control of film morphology during the deposition and crystallization of the perovskite layer. To avoid shunting in such planar structures, a homogeneous and pinhole-free perovskite layer is crucial. Besides surface coverage, the optimization of several other important material parameters, such as material crystallinity and grain structure, could lead to improved electronic properties of the perovskite films and thereby superior device performance.
Generally, the low temperature, solution processed, photovoltaic thin-film materials contain randomly oriented small grains with significant amount of grain boundaries. The presence of high density grain boundary regions creates traps and recombination centers for the charge carriers, and adversely affects the overall carrier transport properties. Accordingly, there is a need in the art for processes suitable for optimizing photoactive layer film morphology with large grains, while simultaneously minimizing the grain boundary regions, for reduced recombination of charge carriers.